The present invention relates to force measurement technique, in particular to a method and apparatus for measuring friction forces on a friction tester.
Tribology is a science of friction, wear, and lubrication on friction surfaces. Many different types of friction testers, tribometers, and other devices for measuring various parameters of friction are known. One such friction tester is disclosed in U.S. Pat. No. 5,795,990 issued to Norm Gitis, et al in 1998. This tester has a lower disk-like test material specimen and an upper rod-like test material specimen or probe which performs orbital motions while being in contact with a stationary lower specimen. A disadvantage of this device is that the upper specimen has a leverage with respect to the point of attachment of the lower specimen, i.e., with respect to its center. As a result, the loading force applied to the lower specimen via the upper probe, as well as the reaction force applied to the probe from the lower specimen create an unbalanced momentum and deformations in the force measurement system. Similar problem occurs in a mechanism for attachment of a bi-directional force sensor in a friction tester, as disclosed in our pending patent application Ser. No. 09/588,054 filed by the same applicants on Jul. 24, 2000.
As shown in FIG. 1, which is a three-dimensional view of a sensor installed in a friction tester for measuring a friction force and other tribological parameters of various materials and lubricants, the device has a sensor 10 formed by a beam 12 flexible in two mutually perpendicular directions for measuring a loading force F1 and a friction force FFR. Both ends of the beam are connected or integrally made with rigid end blocks 14 and 16. The end block 16 is rigidly attached to a loading unit 18 of the tester (not shown), while the end block 14 supports an upper specimen or probe 20 which is maintained in contact with a disk-like lower specimen D. The solid end block 14 has a limited freedom of movement to ensure deformation within the range of measurements. Flexibility of the beam 12 in the X-Z plane, i.e., deformations caused by the loading force F1 are allowed due to the provision of a through slot 21 with notches 22 and 24. The notches have a width wider than the width of the slot 21 thus weakening the beam and making it deformable in the area of the notches 22 and 24. Similarly, flexibility of the beam 12 in the Y-Z plane, i.e., deformations caused by the friction force FFR are allowed due to the provision of a through slot 26 with notches 28 and 30. The notches 28 and 30 have a width wider than the width of the slot 26 thus weakening the beam and making it deformable in the area of the notches 28 and 30. The slots 26 and 21 are partially overlapped within the body of the beam 12 so that the beam can be considered as two deformable parallelograms that arranged in two mutually perpendicular planes. Deformations of the beam 12 caused by the loading force F1 in the X-Z plane are measured by two strain gauges 32 and 34. Reference numeral 34 designates the strain gauge located on the other side of the beam. However, the strain gauge 34 itself is not seen in FIG. 1. Similarly, deformations of the beam 12 caused by the friction force FFR in the Y-Z plane are measured by two strain gauges 36 and 38 located on both sides of the beam near the weakened portion on the other end of the beam (only one of these strain gauges 36 is seen in FIG. 1). In more detail the construction of the beam 12 and principle of its operation during friction testing is described in pending U.S. patent application Ser. No. 09/588,054 filed by the same applicants on Jul. 24, 2000.
In the course of testing, the lower specimen D is brought into rotation, e.g., in the direction shown by an arrow R, and then a loading force F1 is applied to the solid end block 16 whereby the upper specimen 20 comes into contact with the lower specimen D. Application of force F1 causes interaction between the upper specimen 20 and the lower specimen D. The aforementioned interaction generates friction force FFR and reaction force FR. Due to flexibility of the beam 12, these forces cause deformations of the beam 12 which are registered by the aforementioned pairs of strain gauges.
However, the friction force FFR generates unbalanced momentum and torsion deformations in the force measurement system. Such asymmetry results in a number of undesired phenomena, such as occurrence of parasitic vibrations generated during rotation of the lower specimen D, tilting of the upper specimen 20, and as a result, limitation in the frequency of rotation, narrowing of test ranges, and inaccuracy of measurements.
It is an object of the present invention to provide a method and a force measurement apparatus which eliminate an unbalanced momentum and deformation in the force measurement system of the tester, improve accuracy of measurements, broaden the range of test conditions, and prevents such phenomena as parasitic vibrations.